Expanded beads of thermoplastic elastomers, in particular thermoplastic polyurethane (TPU), have elastic and tribological properties and so are useful in a very wide variety of applications. Examples of uses for expanded beads, which are also known as expanded pellets, include production of body protectors, such as helmet shells, knee and elbow protectors, sound and vibration absorbers or packaging, as well as production of gymnastics mats, shoe soles, midsoles, inlay soles or, in the automotive interior, production of, for example, steering wheel, door trim and foot space parts. Expanded thermoplastic elastomer beads are employed with particular preference in shoe soles, midsoles and inlay soles in sports shoes. High elasticity and good homogeneity on the part of the expanded beads are of decisive importance for all these sectors.
Expanded polymers, also known as foamed materials, or foams, and particularly expanded polymer beads, also called bead foams, are known and have been extensively described in the literature, for example in Ullmann's “Enzyklopädie der technischen Chemie”, 4th edition, volume 20, p. 416 ff.
WO 2007/082838 discloses a process for production of expanded thermoplastic polyurethane comprising a blowing agent. A first step of the process comprises extruding a thermoplastic polyurethane into pellets. The pellets are impregnated with a blowing agent in an aqueous suspension under pressure in a second step and expanded in a third step. In a further embodiment of the process, the thermoplastic polyurethane is melted in an extruder together with a blowing agent and the melt is pelletized without a device to prevent foaming. Volatile organic compounds are used as blowing agents in the production via extrusion.
EP-A 0 664 197 discloses the production of expanded thermoplastic elastomers using water as blowing agent in an endeavor to avoid organic blowing agents. An alternative process for producing foams from thermoplastic elastomers by use of carbon dioxide and nitrogen as blowing agents is known, for example from WO 2004/018551. A further process for producing expanded thermoplastic elastomers which recurs to the foam production process described in WO 2004/018551 is also disclosed in WO 2007/044123.
JP 11080408 discloses a process for production of foams on the basis of thermoplastic resins wherein the materials are impregnated with blowing agent under elevated pressure and elevated temperature. Shaped and sized parts are first impregnated and then expanded. JP 2003 261707 similarly discloses a method of producing resinous foams. Pellet material is impregnated, melted, molded and foamed therein. An extruder is employed as molding machine.
WO 2005/105907 describes the production of closed-cell fluoropolymer foams wherein the resins are exposed to inert gases at elevated pressure and temperature and then expanded by reducing the pressure. The resins are crosslinked prior to expansion.
U.S. Pat. No. 4,331,619 describes a foam composition comprising an ethylene-chlorotrifluoroethylene copolymer, a blowing agent and a nucleating agent. It further describes a method wherein the polymer is impregnated by heating to temperatures above its melting point and an extruder is employed for expansion.
WO 00/43193 describes a process for production of microcellular polymer foam from amorphous, semicrystalline or crystalline polymer. It includes a step of saturating shaped bodies with an inert gas at elevated temperature. U.S. Pat. No. 4,473,665 describes the production of microcellular foam by employing extrusion or injection molding techniques.
WO 02/12379 discloses a production process for closed-cell crosslinked polymer foams wherein shaped parts comprising a mixed composition with blowing agent and crosslinking mediator are expanded and crosslinked in one step.
EP 2217644 discloses the production of fluoropolymer foams wherein the polymer material is mixed with blowing agents and additives, shaped and expanded. The shaping can also be achieved by expansion in a mold. The first step is to produce a blend of fluoropolymer resin and blowing agent wherein the blowing agent is in a non-gaseous state or in a state in which gas is not liberated from the blowing agent.
EP 1853655 describes polyamide foams and their method of making. Impregnation and expansion is carried out distinctly above the melting temperature. Shaped parts can be shaped out of the expanded material or be formed directly by expansion in a mold.
In US 2009/0048356 a polymer (preferably in granular form) is exposed to a gas which penetrates into the polymer. This is done at a temperature that is between the glass transition temperature and the melting temperature of the polymer when said polymer is crystalline or semicrystalline, or that is below the glass transition temperature when the polymer is an amorphous polymer. The polymer is then melted to produce foamed articles. In one embodiment, the polymer is completely melted to form a homogeneous melt.
None of the documents known from the prior art, however, discloses a process wherein thermoplastic elastomers such as thermoplastic polyester elastomers, thermoplastic copolyamides, thermoplastic polyurethanes and styrene block polymers are present as beads not only in the impregnating step but also in the expanding step, while a gaseous medium surrounds these beads.